This invention relates to liquid crystals, and more particularly to an improved reflective mode liquid crystal cell construction.
It is already known that a given liquid crystal material exists in the liquid crystal phase over only a limited temperature range. Below this range these materials are generally solids and above this range they are ordinary isotropic liquids. Many of the earliest liquid crystal materials exhibited their liquid crystal characteristics only within a very limited range above room temperature. Through continued development, many materials are now available that exhibit liquid crystal characteristics over considerably wider ranges. Thus, many materials are now available for use as liquid crystals from room temperature to well over 100.degree. C. Therefore, elevated ambient temperatures normally do not present a problem with respect to liquid crystal devices.
On the other hand, there are few materials which exhibit liquid crystal characteristics significantly below room temperature and, of those which do, viscosities are generally so high at low temperatures that response times for devices using them are undesirably long. Accordingly, liquid crystal displays have been limited chiefly to moderate ambient temperature applications, as for example calculator displays, wrist watch displays, etc.
If a liquid crystal display is to be subjected to ambient temperatures colder than room temperature, it may have to be heated to be satisfactorily operative. This is particularly true for displays subjected to the winter temperature extremes of temperate and polar climates. In such climates, exterior visual displays, automobile instrument panels, and the like, would require means for maintaining the liquid crystal display device above unduly low ambient temperatures. Providing such temperature protection is not an easy matter. For commercial practicality, the heating means must be more than just effective to protect against moderately low weather temperatures. For example, in automotive applications it must even be adequate at temperatures as low as minus 40.degree. C. However, it must also be low in initial cost and efficient to operate. In addition, particularly for automotive applications it should be simple, rugged and reliable, requiring virtually no maintenance in use. Still further this heating means must not significantly limit the visual display function of the liquid crystal cell. Thus, separate heated enclosures for the liquid crystal cell are not generally desirable.
We have found a very simple and effective way to provide low temperature protection to a reflectance mode electro-optic liquid crystal display device. It involves a liquid crystal cell having an integrated self-actuating heating element. It does not require a separate enclosure or other composite type of structure to provide low temperature protection. Moreover, in many applications it does not require a separate heater power supply or control, and even shares electrical connections with electro-optic electrodes of the device.